This invention relates generally to techniques for packaging electronic devices and, more particularly, to techniques for packaging hybrid microcircuits in hermetically sealed enclosures.
Hybrid microcircuits are devices in which various types of electronic components, such as semiconductor devices, integrated circuits and passive elements, are assembled together in small protective enclosures to fulfill system size requirements that are not easily met by more conventional packaging techniques. The electronic components, which are usually in chip form, are typically attached to an insulating substrate and interconnected by thick- and thin-film deposited networks, primarily of conductors and resistors. The small enclosures, or hybrid packages, protect the hybrid microcircuits from environmental and mechanical damage and allow the microcircuits to be easily handled.
A typical hybrid package includes a boxshaped metal housing having openings in the side walls or bottom of the housing for leads that electrically connect the hybrid microcircuits to external circuits. The leads are sealed into the openings by a sealing glass which hermetically seals the package and electrically insulates the leads from the housing. The housing is generally fabricated from a metal or metal alloy to provide good heat dissipation for the hybrid microcircuits.
The glass to metal bond between the sealing glass and the metal lead and housing must remain both mechanically stable and hermetic as the package expands and contracts due to temperature changes. Therefore, the sealing glass and metal lead and housing must have similar coefficients of thermal expansion (CTE's). Kovar (ASTM F-15 alloy), a metal alloy of nickel, cobalt and iron, and a borosilicate sealing glass, such as Corning 7052, are often used to fabricate these types of packages because they have about the same coefficient of thermal expansion (47.times.10.sup.-7 compared to 46.times.10.sup.-7). When subjected to heating in an oxidizing atmosphere, Kovar forms a continuous refractory oxide layer on its surface which fuses to the borosilicate sealing glass to provide the hermetic seal. This hermetic seal, which typically has a leak rate of 1.times.10.sup.-9 atm-cc/sec when measured by a helium mass spectrometer, is substantially impervious to diffusion of water vapor and other contaminants.
Unfortunately, Kovar is very difficult to machine and is a relatively heavy metal with poor thermal conductivity. Aluminum produces a much better hybrid package because it is very easy to machine and is a relatively light metal with very good thermal conductivity. However, aluminum has a much larger coefficient of thermal expansion than Kovar, thus preventing borosilicate sealing glasses from being used to directly seal the leads into the openings in an aluminum housing. Kovar feedthroughs, which are preassembled leads having an outer shell or ferrule and center conductor of Kovar with a sealing glass insulator already fused between the shell and center conductor, can be joined to an aluminum housing by soldering. However, the large difference in the CTE's of Kovar and aluminum induce high thermal stresses between the housing and feedthroughs which exceed the capabilities of most solders. U.S. Pat. No. 4,213,004 to Acker et al. discloses a technique for joining Kovar feedthroughs to an aluminum housing by machining a thin annular wall around each opening to allow for expansion of the aluminum housing and then electron beam welding the Kovar feedthroughs to nickel plated edges of the annular walls. However, the thin annular walls require additional machining steps and the use of electron beam welding, which must be performed in a vacuum, requires that the aluminum be plated with nickel. Accordingly, there is still a need for a simple yet effective technique for joining Kovar feedthroughs to an aluminum housing. The present invention clearly fulfills this need.